The present invention relates to multilayered film structures, and in particular multilayered film structures having desirable flexural modulus properties, as well as methods of making the same and products incorporating the multilayered films as a component.
Polymeric films, particularly biaxially oriented polymeric films, are used in a wide variety of applications. For example, biaxially oriented polyethylene terephthlate (xe2x80x9cPETxe2x80x9d) film is employed in a range of applications due to its superior mechanical properties, temperature resistance, chemical inertness, and ready availability.
Biaxially oriented films are typically characterized by a higher flexural modulus than films possessing lower orientation. Biaxially oriented PET films are especially known to have superior stiffness, particularly heavy gauge (or thick) films, e.g, films having a thickness of about 76 micrometers or greater.
Typical applications for biaxially oriented heavy gauge PET films include photographic substrates, reprographic films such as drafting and engineering films, and microfilm. Additional markets continue to emerge for heavy gauge PET films, as well. For example, PET film provides a superior substrate for overhead transparencies, such as those produced on an ink jet printer. The use of film in conjunction with ink jet printing is discussed in greater detail in U.S. Pat. No. 4,775,594 to Desjarlais.
Also, due to its superior flexural modulus, biaxially oriented polyethylene terephthalate is especially well suited for use as a carrier or support substrate in heavy gauge release liners. Heavy gauge release liners are typically employed in the xe2x80x9csecondaryxe2x80x9d label market. Exemplary applications within the secondary label market include fleet marking, e.g., the decals and signage placed on trucks, buses and the like, and labels for appliances.
Labels and decals are commonly provided as multicomponent systems that include a printed label stock adhered to a release liner. In general, print or ornamental designs are applied to an outer surface of label stock, commonly formed from paper or film. The opposing surface of the label stock is coated with an adhesive, for ease in label application. A release liner is typically applied to the exposed adhesive, protecting the adhesive coated label until use.
Release liners generally include a carrier or support substrate and a release layer applied to one or both surfaces thereof. The carrier substrate employed in secondary labels is typically fairly thick, e.g., 127 micrometers or more, primarily because such applications typically require superior stiffness.
To apply the label to a substrate surface, the release liner is peeled away from the printed label stock and discarded. Although the release liner is ultimately disposable, it must nonetheless possess sufficient durability to survive the stresses imparted during downstream processing and further possess sufficient cohesion to separate cleanly from the printed label stock.
Although possessing attractive mechanical properties, biaxially oriented PET films, especially heavy gauge PET film, can be expensive in comparison to other conventional films or paper based constructions. This price differential is especially problematic in highly competitive disposable applications, such as release liners.
Further, processes used to produce PET film can impose limitations on the attributes of the final product. For example, conventional polyethylene terephthalate is generally provided as an essentially clear or colorless film. However, markets for colored polyethylene terephthalate films currently exist and provide tremendous growth opportunity. White films, for example, are of interest as a replacement for the vellum currently employed in drafting films. In addition, ink jet or thermal transfer printing represents a significant market for white PET films, allowing the production of photographic quality computer generated prints. PET film is likewise beneficial in computer generated printing applications due to its superior stiffness and dimensional stability.
Opaque white PET films are commercially available; however, as noted previously, PET films can be expensive in comparison to other substrates. Further, opaque PET films are not commercially available in a wide range of colors. Transparent colored PET films may be produced by dyeing off-line, such as by immersing the film in a solvent based dye-bath. However, solvent dyeing involves the use of hazardous chemicals. Further, the cost structure involved in the manufacture of off-line dyed films is exacerbated, particularly for off-line dyed thick films.
The present invention provides multilayered structures with superior flexural modulus properties at a significantly reduced cost. The present invention further provides economical, environmentally friendly pigmented films likewise possessing the superior stiffness noted for biaxially oriented films.
The present invention is a multilayered structure which includes outer thermoplastic film layers sandwiching an inner layer formed of an unoriented olefinic polymer. The outer film layers of the structure are formed of a polymeric material that is stiffer than the olefinic material forming the intermediate layer. In particular, the outer film layers have a flexural modulus value exceeding 1500 MN/m2, preferably about 3200 MN/m2.
The inner layer has a lower flexural modulus value than either of the outer film layers. Stated differently, the inner olefinic layer is formed of a more flexible polymer. In addition, the inner olefinic layer has a greater thickness relative to the thickness of the individual outer film layers.
The inner olefinic layer can act as a bonding or xe2x80x9ctiexe2x80x9d layer to bond the outer film layers and the inner olefinic layer together to form a unitary structure so that the multilayered structure fails cohesively within a layer rather than adhesively between layers. The resultant multilayered structure of the invention has an overall or cumulative stiffness value exceeding the stiffness value of the intermediate layer. Surprisingly the inventors have found that such a multilayered structure can possess sufficient stiffness for use in applications requiring superior flexural modulus values, such as release liners and the like, despite the presence of a relatively flexible interior material.
Advantageously at least one, and preferably both, outer film layers are formed of biaxially oriented film. The outer film layers can be produced from a variety of polymers, including polyesters, biaxially oriented polypropylene, polycarbonates, polystyrene, and copolymers, terpolymers and blends thereof. In one currently preferred embodiment, at least one, and more preferably both, of the outer layers is formed from polyethylene terephthalate. The thickness of each the outer layers advantageously ranges from about 6 to about 75 micrometers, preferably from about 6 to about 51 micrometers, and more preferably from about 12 to about 36 micrometers.
Intermediate layers suitable for use in the multilayered structures include any of the types of olefinic polymers known in the art to be useful in extrusion coating. Exemplary polyolefins include homopolymers, copolymers, and terpolymers of an alpha-olefin having a carbon number ranging from about 2 to about 10. Exemplary alpha-olefins include high density polyethylene, low density polyethylene, polypropylene, polymethylpentene, and copolymers, terpolymers, and mixtures thereof. The intermediate layer can range in thickness from about 50 micrometers to about 100 micrometers. The intermediate layer may further contain one or more pigments. The presence of white pigments in the intermediate layer can be particularly advantageous in various end uses.
The multilayered structures of the present invention can be employed in any conventional thick, e.g. heavy gauge, film application. For example, a release coating can be applied to one or both surfaces of the multilayered structure to provide a release liner. The multilayered structures can also be coated with compositions that render the structures suitable for use in graphic arts, photographic, reprographic, and print receptive applications.
The multilayered structures of the present invention can be prepared using an extrusion coating or laminating process. Specifically, a surface of a film layer can be activated, either off-line or in-line, using conventional techniques such as corona treatment, chemical priming, flame treatment and the like. A surface of a second film layer can be similarly activated, also using conventional techniques. A suitable polyolefin resin is then heated and extruded as a molten film or sheet onto the activated surfaces of the film layers simultaneously to form a film/olefin structure. In an alternative embodiment, the polyolefin resin is initially extruded onto the activated surface of a first film layer. The activated surface of a second film layer is then placed into cooperating face-to-face relationship with the exposed olefinic surface to sandwich the olefinic material between the outer films layers and form a laminate structure. The resultant multilayered composite structure can be directed through a nip formed by cooperating pressure and chill rolls to solidify the molten polyolefin resin, thus forming a unitary multilayered composite structure. The product can then be directed to a winding roll or immediately directed to additional downstream process (such as in-line application of a release agent, etc.).